If people were allowed to enter from the back of the bus as well, then the people who had been waiting longer at the front of the line might not be able to get on that bus, as it would have filled up from the back. That would hardly be fair.
Disney does have some guidelines about the size of strollers allowed in the parks:
Q. Are there any personal items I should not bring with me into the Theme Parks?
A. Items that you may not bring into the Theme Parks include, but are not limited to:
Items with wheels, such as wagons, skateboards, scooters, inline skates, shoes with built-in wheels, two-wheeled or three-wheeled conveyances,
strollers larger than 36" x 52", suitcases, coolers, or backpacks with or without wheels larger than 24" long x 15" wide x 18" high (coolers required for medication may be stored in a locker or at Guest Relations), and any trailer-like object that is pushed or towed by an
ECV wheelchair or stroller
Alcoholic beverages
Weapons of any kind
Folding chairs
Glass containers (excluding baby food jars and perfume bottles)
Pets (unless they are service animals)
In Disney's Animal Kingdom® Theme Park (for the safety of the wildlife), balloons, straws and drink lids are not permitted
36" X 52" is pretty humongous, so WDW is pretty well set up for the standard double stroller. Just don't expect it to fit into too many giftshop aisles.